Fluid flow velocity measurement instruments, for example an ultrasonic Doppler submerged A/V flow meter, use the existence of reflective targets for estimating fluid flow velocity. An underlying assumption on which many meters are based is that the various targets in the fluid are moving at the same speed as the fluid. The fluid flow velocity is often not uniform because, considering an open channel conduit as an example, there is a velocity profile in which fluid closest to a channel wall flows more slowly than fluid towards the center of the channel.
To the extent that targets in the fluid, for example bubbles or particulate matter, are uniform in their size and disposition, the frequency spectrum of ultrasound reflected from a large ensemble of moving targets in the fluid may represent a statistical velocity distribution for the channel, and thus be leveraged to provide an estimate of the fluid flow velocity. The average velocity, which in turn may be used to calculate flow rate, may then be determined by computing the centroid of the Doppler spectrum.